Nowadays, a lot of effort is put in optimising the cost of energy of wind turbines. Cost of energy comprises three main items, i.e. capital expenditures (capex), operation and maintenance costs (O&M costs) and annual energy production (AEP). To improve the cost of energy of a particular wind turbine type, one, two or each of the different items can be tackled.
One way of handling the O&M costs is by focusing on improving and simplifying servicing of the wind turbine. Servicing, e.g. maintenance or component replacement of wind turbine drive train components such as a gearbox and/or generator, is in many cases a difficult and expensive activity. Therefore, wind turbine designs should not only be reliable such that servicing activities can be limited, but should also be service friendly, such that servicing, when necessary, can be performed easily and at low cost.
Nowadays, a lot of effort is done for finding solutions for making servicing activities of wind turbines easier and less expensive.
WO 2010/130717 describes a wind turbine comprising a hub 1 carrying one or more blades, a frame 2 and planetary gearing for transmitting the torque of the hub 1 (see FIG. 1). The hub 1 is rotatably mounted upon the frame 2 at or near a distal end thereof by means of bearings 3. The torque of the hub 1 is introduced into the planetary gearing through a planet carrier 4 which is located at or near the distal end of the frame 2. The hub 1 may therefore be connected to the planet carrier 4 at various connection points 5 around the circumference of the hub 1. The planet carrier 4 may be one integral element or may be formed with a first part 4a and a second part 4b connected to each other. The connection may be formed by simple fasteners such as screws or bolts. Alternatively, the connections may comprise at least one elastic element such as flexible bushings. Planet shafts 6 are rotatably supported at both ends within the planet carrier 4. An annular gear 7 is arranged around planet gear wheels 8. The torque of the hub 1 is in this way transmitted from the planet carrier 4 to a sun gear 9 mounted on an output shaft 10 of the first stage. The planetary gearing comprises a second stage comprising a planet carrier 14 carrying a plurality of planet gear wheels 18 upon planet shafts 16. First stage output shaft 10 functions as input shaft for the second stage. The torque is transmitted through the planet carrier, which is formed of separate elements 14a and 14b. The planet carrier 14 is rotatably mounted through bearings 13 upon a support structure 12. Planet gear wheels 18 rotate within second stage annular gear 17, whereas second stage sun gear 19 is mounted upon second stage output shaft 20.
The drive train described above is relatively compact, and is substantially completely housed within the frame 2. Repair and installation of gearing is relatively simple, because the configuration allows easy access to the remainder of the gearing by simply removing the planet carrier 4 from the hub 1.
Another embodiment described in WO 2010/130717 is illustrated in FIG. 2. In this embodiment, the hub 1 comprises an extension 1a, connected to the hub 1 at various connection points 5. The planet shafts 6 are cantilever mounted and the planet gear wheels 8 comprise double gearing, first gearing 8a meshing with sun gear 9 and second gearing 8b meshing with annular gear 7.
First stage output shaft 10 serves as second stage input shaft and carries second stage planet carrier 14. Planet carrier 14 is rotatably mounted through bearings 13 in support structure 12. Planet gear wheels 18 mounted upon planet shafts 16 transmit the rotation to second stage sun gear 19 and second stage output shaft 20. Second stage output shaft 20 is rotatably mounted through bearings 33 in the housing of the generator 30. The generator 30 comprises a generator rotor 31 and a stator 32. Generator rotor 31 is driven by second stage output shaft 20. The generator housing is integrally formed with frame 2, upon which hub 1 is rotatably mounted through suitable bearings 3.
In the embodiment illustrated in FIG. 2 no part of the planetary gearing is mounted within frame 2. Instead, the components of the second stage of the planetary gearing are mounted within a support structure 40 arranged within hub 1, forward of frame 2. An advantage of the arrangement with the forward support structure 40 is that both installation and maintenance of the planetary gearing is facilitated; easy access to the planetary gearing is ensured.
Support structure 40 may be connected to frame 2 through a flexible connection 15 which can only transmit axial torque. The advantage of a coupling 15 that only transmits torque is that support structure 40 and also the planetary gearing carry no substantial bending loads. All cyclical loads due to e.g. weight of the hub 1 are transmitted only to frame 2. This may reduce the fatigue loads on the gearing and increase its life time.
In the embodiments described in WO 2010/130717, the planet carrier can simply be removed so as to allow easy access to the remainder of the gearing. However, when the complete gearbox has to be removed, it still may be an expensive and time consuming action as the complete gearbox still cannot easily and completely be removed.
Another disadvantage of the embodiments described in WO 2010/130717 is that, due to rotor loads, the frame 2 can deform, which can have a negative effect on the bearings 3. This can be solved by making the frame more stiff, which will increase the manufacturing cost, and thus the capex, of the hub 1.
Another example is described in U.S. Pat. No. 6,232,673. This document describes a hub 101 of a rotor employed in a wind-power plant (see FIG. 3). The hub 101 is provided with accommodations 102 for blades and is accommodated in a large-scale roller bearing 103 which is attached to a rotor support 104. The rotor support 104 is connected to a mast (not shown) by way of an azimuth bearing 105. The roller bearing 103 has a stationary outer ring rigidly fastened to the rotor support 104 and a rotating inner ring 107 onto which the hub 101 is screwed. The wind-power plant also includes a transmission in the form of a two stage planetary gear comprising an input stage 109 and an output stage 110. The axle of the sun wheel in input stage 109 constitutes the gear's output shaft and is coupled to the shaft of a generator 111.
The inner ring 107 in roller bearing 103 is connected to the hollow wheel 113 in input stage 109. This hollow wheel 113 is connected to another hollow wheel 114 in output stage 110 in the planetary gear's rotating housing 115. Planetary support 116 forwards the reaction torque deriving from the planetary gear into the rotor support 104.
The generator 111 is screwed onto the stationary planetary support in input stage 109 by which the planetary gear and generator 111 are combined into a single drive-train module. This module is connected to rotor support 104 by way of vibration suppressors 117. The wind-power plant furthermore comprises mounting rails 118 for installing and removing the module from the hub 101.
The wind-power plant's modular construction allows the planetary gear and its subassemblies and generator 111 to be removed from the mast by a crane individually or as a whole and be replaced. This can, especially for offshore applications, reduce the expense of maintaining a wind-power plant.
For removing the planetary gear and generator 111 the nacelle has to be opened, such that a crane can be attached to the planetary gear and generator 111. Furthermore, for being removed, the planetary gear and generator 111 have to be displaced through the nacelle to the back of the nacelle. Consequently, other parts, such as for example a lubrication system or a converter, which are present in the nacelle also have to be displaced. Moreover, the planetary gear cannot be removed without having to remove the generator 111.